Do collard greens taste similar to anything else, as a reference point? I don't think I've ever had any before. I've always had a negative opinion of them, which is nothing more than an unfounded bias since I've never tried them! Just by looking at your picture of them, my brain is coming up with a bitter, Brussels sprouts-like taste in my imagination. But I like Brussels sprouts!
Maybe I did have collard greens before, but my brain is not remembering it if i did. My memory for things like that has always been pitiful.
They don't taste much different than spinach.